1. Field of the Invention
The present invention relates to a carrier for fluid treatment composed of a polyolefin-based foam and a method of making the same, the carrier providing excellent fluid treatment performance such as water-sinking properties.
2. Description of the Related Art
Heretofore, foams composed of polyolefin resins or the like have been widely used as carriers for microorganism immobilization for water treatment such as BOD treatment, nitration treatment, and denitrification treatment. However, such foams exhibit poor water wettability and water-sinking properties because they are composed mainly of hydrophobic resins and contain many closed cells and half-through cells. Therefore, there is no carrier for water treatment composed of a polyolefin-based resin foam having sufficient water treatment capacity.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2004-224904 suggests a method of making a crosslinked polyethylene-based resin by two-stage foaming under a batch method, wherein a polyethylene-based resin is mixed with a crystallization promoter, a foaming agent, and a cross-linking agent to produce a foaming resin composition, the composition is filled in a closed mold, heated under pressure to produce an intermediate foam, subsequently the intermediate foam is heated under normal pressure to decompose the residual foaming agent and cross-linking agent, and thus a foam is obtained.
However, the making method suggested in JP-A No. 2004-224904 is a batch method with a low production efficiency, and the foam produced in the foaming step is inhomogeneous. In addition, two-stage foaming is inefficient, and the resultant foam floats on water because it contains many closed cells and half-through cells, and thus is unsuitable as a carrier for water treatment or microorganism immobilization.
Japanese Patent Application Laid-Open (JP-A) No. 11-315161 suggests a method of making a crosslinked polyolefin open-cell foam, wherein polyolefin is mixed with a foaming agent and a cross-linking agent to produce a foaming cross-linking composition, and the composition is heated and foamed in an unclosed mold to produce a foam, and then the foam is mechanically deformed thereby communicating cells with each other.
However, the making method suggested in JP-A No. 11-315161 is inefficient because it involves two steps of producing a foam and mechanically deforming the foam thereby communicating cells with each other. Examples of common techniques for producing an open-cell foam include a drop-foaming method and a sintering method of making a porous body. However, these methods are inefficient.
Japanese Patent Application Publication (JP-B) No. 3143412 suggests an invention of a microorganism immobilizing carrier for fluidized-bed including an extruded foam composed mainly of a polyolefin-based resin, wherein the extruded foam contains open cells communicating with the foam surface and closed cells not communicating with the foam surface, and the proportion of the open cells in the extruded foam is from 20 to 70%.
The microorganism immobilizing carrier according to the invention suggested in JP-B No. 3143412 contains open cells composed of through cells, at least two points of which communicate with the foam surface, and half-through cells, a single point of which communicates with the foam surface. In any case, a formation of open cells requires two steps, or foaming and communicating cells with each other, which results in low productivity. In addition to closed cells, if the foam contains half-through cells in the open cells, the foam floats on water and thus unsuitable for uses as a microorganism carrier. If the proportion of the open cells is from 20 to 70%, the expansion ratio is from 1.25 to 3.3. The expansion ratio is too low and the production efficiency is low. Furthermore, the expansion ratio achieved by extrusion molding is about 10 for closed cells, but at most 3 for open cells.
For example, Japanese Patent Application Laid-Open (JP-A) Nos. 5-228980 and 5-230254 describe the inventions of the methods for continuous production of foams composed mainly of a polypropylene resin by extrusion-foam molding. According to the inventions suggested in JP-A Nos. 5-228980 and 5-230254, the expansion ratio is 10, but the foams are composed of closed cells, and thus unsuitable for carriers for water treatment or microorganism immobilization.
As described above, highly expanded foams composed of open cells (water-sinking highly expanded foams) can be produced only by a batch method, and continuous production of such foams is believed to be impossible.